If you are doing failure analysis, you need to be able to tell if the component was made by the specified process. I actually bought a pair of pliers once that broke the first time I used it. Unbelievably, it was not forged steel but a zinc die casting! I could tell from the obvious (visible to the naked eye) voids that were revealed by the fracture event.
But it’s not always so “self-evident.” Sometimes we have to go to the trouble of making a metallographic cross section to examine the microstructure or using a scanning electron microscope to see what process steps were used to make the object we’re looking at. That said, when you do have access to a microscope, it is usually fairly easy for the experienced materials engineer to distinguish a casting from a wrought product.
Many materials start out in liquid form. Castings and injection-molded parts are two examples. But even forgings usually start out as a cast ingot if made by a traditional process. And many powder-metal materials are atomized from a molten state. Metal powders are the foundation of additive-manufacturing processes, including both thermal and cold spray. Hot- and cold-rolled steel, aluminum and copper all start out as molten material that is poured into molds for solidification prior to the rolling processes.
Figure 1 shows an example of a casting cross section (metallographic specimen) with a curvy black line. We see rounded features with a different “lacy” structure filling in between the ovals. With such rounded features, it is a good working assumption that you are looking at a casting. But the heavy black line is not a true crack. No cracks have that shape. This means it was a discontinuity that was present at the time that the casting solidified. If we could separate the two pieces and look at them directly, rather than in cross section, we would not find any true fracture features (no matter what magnification we used). We’d find “non-fracture” features.
Sometimes the rounded features don’t look rounded. If they are dendrites, a type of crystal that starts to grow in a cold area (the surface of the mold, for example) and extends itself in (toward the center of the mold, for example), you may see something like Figure 2. There are patches of rounded features mixed with long features. The rounded features look similar to the ones in Figure 1. But the long and circular features are all the same type of structure. Some happened to be cut along their length instead of in a transverse manner. The long “branches” often have smaller branches, or “secondary dendrite arms,” associated with them.
Figure 3 is a scanning electron micrograph, and it is showing a topological view of some dendrites from a steel casting. The word dendrite is from a Greek word meaning branch. You can see why these structures have this name.
Figure 4 Is an SEM photo of a “crack” (not a cross section) surface taken from a casting. This is a fairly high-magnification image, and we don’t see any long dendrites with distinctive secondary arms, such as depicted in Figure 4. This was likely from a die-casting process, which solidifies very rapidly.
The fine features, the tiny pockets and bumps are telling one story, while the larger-scale features – the rounded ovals seen in the lower left corner and running on a diagonal up toward the right – are telling another. The larger-scale features indicate it’s a casting. In other words, we are looking at a structure that formed on solidification. The shape it has is the shape it had when it first solidified (not counting machining, where material might have been removed without changing any structures of the remaining material.) But it is possible that the fine features could change were the component subject to heat treatment.